The growth of interest in the first world war and its poets seems unstoppable. There have been three biographies of Isaac Rosenberg, three of Robert Graves and two (soon to be three) of Siegfried Sassoon. Yet Wilfred Owen, the most famous voice, has been oddly neglected. There's only been one full-length life of him, published almost 30 years ago. Sources have remained untapped and questions unanswered and plenty of new material has come to light.

Many stories about Owen need to be substantially retold: about his early life and education, and his time as a parish assistant at Dunsden, where he struggled in vain to reconcile religion and "the flesh". His two years as a tutor in France have not been fully understood, nor has his friendship with the old decadent poet Laurent Tailhade, who was far from being the pacifist he's usually claimed to be.

It's no longer enough to be told that Owen fought "somewhere near Beaumont Hamel" in January 1917. The Public Record Office has trench maps, orders and reports, as well as the diaries kept by every fighting unit from battalion up to army level. In northern France you can walk up to Redan Ridge from Beaumont Hamel, struggling against an "iced east wind" that claws you as it clawed Owen and the 2nd Manchesters long ago. Then suddenly you're facing Serre across the empty hilltop and understanding at last why he and his platoon had to lie flat on the snow, unable to move. One man froze to death. Another day would probably have killed the rest - here, on this relentless clay, among these huge flints that must have splintered viciously in machine-gun fire. Owen remembers the ordeal in his poems "Futility" and "Exposure": "Pale flakes with fingering stealth come feeling for our faces - / We cringe in holes, back on forgotten dreams..."

Serre is mentioned only once in Owen's letters, and consequently never, until very recently, in the literature about him. Yet the place was immensely important for his later work. The village was the Germans' last unconquered stronghold on the Somme front. Thousands of men had died trying to take it, and the ground was littered with corpses, "the most execrable sights on earth", Owen said. "In poetry we call them the most glorious." Poets don't call them glorious any more, and he more than anyone has been responsible for the change.

Owen has often been thought of as a wimp; he is supposed to have been mollycoddled, a mother's boy, but despite the protestations of affection in his letters he wasn't too keen on going home during the last five years of his life. He was a small man, only 5ft 5 when he enlisted, but as an experienced teacher and good amateur actor, he never had any difficulty in commanding troops. His training in 1916 was probably the best available in the army (again, there are records) and he took it very seriously. By the time he got to France he was a tough, efficient and well-regarded officer. He coped very well - his experiences were far worse than anything endured by, for example, Sassoon - but in the end the constant horrors and a near-miss by a shell took him to the edge of breakdown.

The temporary CO seems to have made some comment implying cowardice. Not a formal accusation, but it was the last straw. Some people have said Owen was in danger of court martial and a firing squad, but that's nonsense. Senior army men, including some doctors, were inclined to regard shell-shock as "funk", but by 1917 the condition was well recognised. Treatment had to be immediate, as Owen's was. He came under the care of a top specialist, William Brown, and was sent on to Craiglockhart War Hospital, where his doctor was Arthur Brock. Brock remained a mere name for many years in Owen biography, but he was a remarkable character. His theories and methods were crucial to Owen's recovery and ensuing poetry.

Owen's cure was completed by his friendship with Sassoon at the hospital. Sassoon showed him that front-line realities could and should be subjects for poetry and he gave him the political motive for writing. There's no point in protesting against a war if you believe in it, as Owen had done so far, but Sassoon passed on new ideas from - ironically - civilian pacifists: the politicians had abandoned the honourable, limited aims of 1914 in favour of smashing Germany completely and grabbing colonies. If only people could be made to see the truth, Sassoon argued, the appalling nightmare could be ended by negotiation. That was probably a false hope in 1917, but it was his motive for attacking the public conscience and it became Owen's.

It is no coincidence that the two poets who criticised the war most powerfully were both gay. The home front in 1917-18 was intensely reactionary: Asquith's Liberal government had fallen, public opinion was being shaped by jingoistic thugs like Horatio Bottomley. Secrecy became an important theme in Owen's poems. He and Sassoon were well placed to take a critical view of society. Their feelings for young men could be put to hidden yet very public use, helping to fire their poetry and combining readily with protest, realism and pity - and with an officer's duty of "care for the men". Sassoon introduced Owen to Robert Ross, Oscar Wilde's champion and first male lover. By the spring of 1918, Owen had literary friends in London, and his self-confidence was more secure than ever.

Owen's homosexuality has often been sidestepped or regarded as insignificant. His brother Harold, whose memoirs have been a major source for Wilfred's early life, resolutely denied it, and for years after Harold's death in 1971, his image of Wilfred persisted: "monk-like", too sheltered and too immersed in poetry to take much notice of sex. It is an image hopelessly at odds with the lesser-known poems, with their delight in male beauty and their brazen enough references to rent boys and street cruising.

Harold Owen's stories often sound improbable, and it's strange he was believed for so long. His inaccuracies did not just come from an erratic memory. He went carefully through his brother's letters, burning, painting out or mutilating anything he thought should be suppressed. These losses were far more severe than has been generally realised. The surviving letters show there are gaps at many crucial points. We shall never know what the "momentous affair" was at Bagnères in 1914, or the exact circumstances of Owen's being sent to France in 1916 and 1918, where he was killed aged 25, on November 4, a week before the armistice. It's not only family gossip and worries about job prospects and money that have gone, but also many pages about friends, books and poetic ambitions.

Perhaps a little about sex has gone as well, but Owen was writing to his mother and the modern notion that he came out to her is dotty. There would have been more about sex in letters to Sassoon, but Sassoon himself burned some of those. And Owen was perhaps the most destructive censor of all, leaving his mother with a whole sack of papers that she had to burn unread if he was killed. She felt obliged to do as he wished. Many of those papers were probably letters to him (very few such letters survive) rather than anything by him, but perhaps there were unfinished poems, too, including drafts of the unwieldy epic about Perseus that he had struggled with for years.

With much of the evidence missing, Harold must have hoped his history of the family would stand unchallenged. But local records show that Owen did not live in a slum in Birkenhead, and he was not sent to ghastly boarding schools. The parents did not have to find "considerable" fees for Owen's education in Shrewsbury, because he worked for four years (not one term) as a pupil-teacher, getting free tuition and even earning a small salary. Harold's defenders insist he was an honest, well-meaning man, and his memoirs are fascinating, if very far from being a reliable, objective record. He can never have imagined that anyone would trawl through school registers, pass lists, street directories, census returns, parish magazines, newspapers, army files, local memories, or forgotten letters and poems.

Tiny details, maybe. But bringing the portrait into focus depends on sharp detail. I hope that the Wilfred Owen who emerges is more complex, interesting and impressive - and no less loveable - than the man we thought we knew before.

· Dominic Hibberd's book, Wilfred Owen, A New Biography , is published by Weidenfeld & Nicolson this month at £20.